I understand it's legal but I was always wondering how this could be morally acceptable and why developers don't lobby against this. I understand the view of the employer: if you work on a project A at work, and on a n open-source project B that is direct competition to project A, you might be acting against the interests of the company. So to save everybody's time, they claim the right to everything and it's done.

However, this is hurting you as developer when you work on unrelated projects. Your work for the community is impeded (e.g. in the case of Google you need to go through a lot of bureaucracy) or completely impossible. In theory you can write some code for yourself, but what motivation do you have, knowing that your employer will be its owner anyway? You will have less motivation to explore new areas of interest for the same reason. It's terrible for personal development.